A couple (literally 2) male guppys or 3-5 male endlers would really be your only readily available choice as far as fish.

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They will eat the shrimp. And in such a small tank the males will be aggressive, you need a 10 gallon minimum to have a all male colony. I wouldn't add anything except shrimp. The thing is it's a cube and schooling fish need a long rectangular tank. You could get a betta just make sure you have somewhere else to put him if he goes after the shrimp since every betta is different.

Guppies won't work in a four gallon. Ten is minimum for three males.... endless wouldn't work either...

Other thing of concern is the two varieties of rcs... they will breed with each other and you will end up with a brown mess (yes, I tried it). I have a betta with rcs, three in fact. One leaves them completely alone, one leaves the adults alone but eats some of the juvies, one hunts them all. I've even seen my guppies eat a rcs whole (was actually funny, spotted a guppy looking not so great, when I walked over to the tank the rcs head was still hanging out of its mouth... no idea where she got the idea to eat it, likely she won't do again, didn't seem comfortable)